In a mobile radio communication system, communication can be continued without interruption by performing a handover between a mobile station and a network when the mobile station moves between base stations.
Here, the handover is a technology by which a mobile station performing communication with some base station changes the communication partner when the distance to the base station increases (the receiving level from the base station decreases) and the distance to another base station decreases (the receiving level from the other base station increases). Thus, the mobile station constantly measures the radio environment to search for a base station with which communication can be performed with better transmission quality.
A communication area covered by one base station is normally an area of several km in radius and a plurality of base stations is set up in such a way that communication areas thereof are adjacent to each other without gaps. A communication area covered by one base station may be called a cell, and particularly, a communication area of the above size may be called a macro cell.
A method of setting up macro cells to cover a communication area is considered to be effective in terms of cost because a wide range can be covered by a small number of base stations. On the other hand, when communication is performed using a relatively high frequency band such as the 2 GHz band, there is a problem that conditions in which radio waves do not reach areas behind buildings, indoors, and underground are more likely to arise.
Thus, an attempt has been made to solve the above problem by further setting up, in a communication area of a macro cell, a base station (for example, an indoor base station) covering a communication area whose range is smaller than that of the macro cell. The cell covering a communication area smaller than that of a macro cell may be called a micro cell or pico cell. For example, a handover when a macro cell and a micro cell or pico cell are used with an overlapped communication area is disclosed by International Publication Pamphlet No. WO 2002/104056 and Japanese Patent No. 2693924.
An ultra-small cell, also called a femtocell, forms a cover area still smaller than that of a micro cell or pico cell. The communication area covered by a femtocell is about 10 m in radius, which is particularly suitable for covering a narrow indoor area, for example, an area of a home.
When a femtocell is set up indoors, for example in a home, a broadband line such as ADSL and FTTH can be used for the connection between a base station and a core network and thus, there may be no need to set up a dedicated line used normally between a base station and a core network so that infrastructure costs can be reduced.
When a mobile station performs a handover from a macro cell to a femtocell, the mobile station searches for a communicable femtocell located within the range of the macro cell.
When the search for a femtocell is being conducted, the base station forming the macro cell repeatedly notifies the mobile station of information (broadcast information) about cell synchronization in a specific short period. Here, information contained in the broadcast information includes an operator code, cell ID, information serving as criteria for determining cell communication quality measurement, adjacent cell information (a cell frequency, cell ID, and the like) and the like.
If, on the other hand, a plurality of femtocells equal to or greater than a certain number is present in a macro cell, the base station forming the macro cell may not be able to transmit broadcast information about all of the plurality of femtocells in the specific period. If a notification period is set longer in accordance with the amount of information transmitted by the base station forming the macro cell, it takes an accordingly longer period for the mobile station to receive information used for handover so that communication may be cut off by exceeding the timing at which a handover can be performed.